An electrode consisting of several very thin foil-like elements with parallel arrangement is described in DD-PS 285 125 and WO 91/00379 A1. Between the adjoining elements of one electrode there is a narrow gap which causes the capillary effect.
Very thin fine-structured electrodes of this kind have a high degree of efficiency. On one side the surface involved in the reaction is very large while on the other side the capillary forces ensure an intensive electrolyte exchange and the discharge of the gas formed cross-wise through the electrode into the degasification chamber each side of the reaction surface. This degasification chamber must be several millimeters deep depending on the structural height of the electrolysis cell and thus takes up space in the order of magnitude of the electrode itself.
It is not possible to dispense with the degasification chamber while retaining the capillary conditions which are advantageous, per se, between the electrode elements because the electrode elements which are structured substantially across the electrode plane do not allow hardly any vertical material transport. DD-PS 285 127 and DD-PS 285 128 describe electrode elements with different profiles, They serve to maintain the capillary gaps through simple positioning of adjoining electrode elements and also to provide their electrical contact.
However even when using unprofiled electrode elements and a vertically permeable capillary gap electrode it would not be possible to dispense with the degasification chamber. The gas bubbles cannot rise unhindered in the capillary gaps, They would coagulate into large bubbles which take up the entire width of the electrode and then "blow out" the electrolyte from the electrode.
DD-PS 285 122 discloses an electrolysis cell which without using a membrane which separates the anode from the cathode chamber ensures separation of the gases thus formed. To do this the electrodes which are made from thin foil-like elements with capillary spacing are sealed sufficiently well from the cell wall enclosing same. The distance between the electrodes must correspond to at least three times the bubble release diameter, Furthermore in the upper cell area a partition is provided which prevents the gases formed in the anode and cathode chambers from mixing in the upper electrolyte-free chamber.
FR 22 44 836 shows an electrolysis cell with an electrode made of several parallel oblong parts wherein the adjoining parts should have a distance in the range of 1.5 to 10 mm, Thus even with an electrode spacing of 6 mm maximum and a current density of 2 kA/m.sup.2 the electrolyte is reliably prevented from being blown out from the reaction chamber. However there is the drawback that even with a retarded foam formation the gas bubbles remain for a long time in the reaction chamber between the electrodes whereby the electrical conducting capacity of the electrolyte is noticeably reduced.
An electrode of porous materials is described in FR-PS 2 308 701. It consists of a self-supporting compound of two porous layers wherein the surface of the electrode on the reaction side is connected to an insulating fireproof oxide layer whose pore radii have at the most a tenth of the pore radii of the electrode material. The oxide layer is hydrophillic and is permeated by the electrolyte.
The gas formed by the electrode in the large pores cannot readily pass into the smaller pores of the oxide layer so that the gas emerges at the back of the electrode. Furthermore it is proposed to provide the back of the electrode with a hydrophobic gas-permeable layer in order to allow the gas to enter directly into a degasification chamber, separate from the electrolyte.
This electrode does indeed avoid any gas bubble loading of the electrolyte between the anode and cathode, but this advantage is at least partly given up through the insulating oxidic layer. A particular disadvantage however is that such porous materials have little flexibility and are therefore not suitable for setting up an all round electrode zero spacing. Their relative rigidity is often a cause of damage to the partition system, such as membranes and diaphragms. There are also stresses to be taken into account during the operation of the electrolysis cell. From EP-A-O 159 138 an electrolysis cell is known using heavily structured but closed electrode surfaces. Its structure is characterised by vertically aligned first main channels spaced from each other by several centimetres for supplying the electrolyte, and by second main channels provided in between same for discharging the reaction products, more particularly the gases. Between the main channels extend substantially finer structured secondary channels which run horizontal and parallel to the diaphragm and supply the reaction surface with electrolyte. A so-called zero spacing of the electrodes prevents the occurrence of vertical material currents in this area. Thus, fluctuations in the temperature and pressure lead to considerable strains and relative movements which can result in the formation of cracks in the oxide layer which are places of undesired gas precipitation in the reaction chamber of the electrolysis cell between the electrode and partition system.
EP-A-O 159 138 discloses an electrolysis cell using heavily structured but closed electrode surfaces. Its structure is characterised by vertically aligned first main channels spaced from each other by several centimeters for supplying the electrolyte, and by second main channels provided in between same for discharging the reaction products, more particularly the gases. Between the main channels extend substantially finer structured secondary channels which run horizontal and parallel to the diaphragm and supply the reaction surface with electrolyte. A so-called zero spacing of the electrodes prevents the occurrence of vertical material currents in this area.
This electrode construction leads to strong gas enrichment in the channels provided for discharging the gas and which cover a significant part of the reaction surface. The electric resistance thereby rises considerably which leads to an impairment of the degree of efficiency of the electrolysis cell. A disadvantage here is also the solid design of the electrode which requires high standards when producing extremely flat reaction faces in order to achieve the zero spacing.
As opposed to most electrolysis processes where the electrodes are completely covered by the electrolyte the fall film electrolysis uses an electrolyte current which moves from top to bottom and thus against the gas bubble flow. In order to ensure sufficient wetting of the electrodes a considerable electrolyte excess must generally be used which is supplied finely distributed at the top of the electrodes. The use of the fall film electrolysis on capillary gap electrodes seems basically suitable to solve the problem of the all round even electrolyte distribution. However at least with those electrodes with vertical gas transport there is the danger of a "blow out" of the electrolyte. Simply because of the two volume flows, electrolyte and gas which are maximum in the upper electrode area and directed against one another, the use of the fall film electrolysis on electrodes with capillary-spaced electrode elements appears problematical.